1. Field of the Invention
The present invention relates to a method for calibrating a radiation detector, and to a particle therapy system.
2. Description of the Related Art
A scanning method is becoming widespread in particle therapy. The scanning method is a technique used to divide a target region into very small segments, called spots, and irradiate each spot independently with a narrow beam having a small diameter of 1σ=3 to 20 mm. The irradiation is stopped once a predetermined dose has been delivered to the spot, and after that, a next spot is scanned with the same beam. Scanning magnets are used when the beam is to be scanned in a direction perpendicular to a direction in which the beam travels (hereinafter, this scanning direction is referred to as the lateral direction, and likewise the traveling direction of the beam, as the depth direction). After the predetermined dose has been delivered to a certain depth in all spots, the beam is scanned in the depth direction. In this case, energy of the beam is changed using an accelerator or a range shifter. Finally, a uniform dose is delivered to all spots, that is, the entire target.
During these scans, an operator of the particle therapy system measures the scan positions, Bragg curve profiles, and dose distribution patterns of the beams, analyzes measurement results, and judges whether the system is correctly adjusted.
Traditionally known radiation detectors are constructed to include a plurality of parallel-plate ionization chambers formed in stacked form in the depth direction, and to be able to measure depth dose distributions of particle beams at one time, and examples of these devices are described in JP-2011-153833-A and M. Shimbo, et. al., “Development of a Multi-layer Ion Chamber for Measurement of Depth Dose Distributions of Heavy-ion Therapeutic Beam for Individual Patients”, NIPPON ACTA RADIOLOGICA 2000 60 274-279.
Each of the stacked ionization chambers usually varies in dimensions and in performance characteristics, and even if the target is scanned with beams equal in the number of particles and in energy level, results output from a signal-processing unit will usually differ for each ionization chamber. This is because thickness of the ionization chamber in the depth direction varies from chamber to chamber, and because conversion from an electric charge into a digital value in the signal-processing unit tends to vary in efficiency. For example, a 10% variation in the thickness of the ionization chamber appears as a 10% variation in the charge output. These variations are due to machining errors, and are therefore difficult to avoid. Traditionally, these variations have been calibrated by comparison with measurement results output from a water-phantom dose detector.